The present invention relates to a pump turbine in which a pump or a turbine can be operated by changing the rotating direction of a runner.
A pump turbine is designed in such a manner as to be operated as a pump or a turbine and to satisfactorily exhibit a centrifugal force action for pumping during a pumping operation so as to achieve a predetermined high pumping head. However, the design for the pumping operation has an adverse influence on a turbine operation, that is, discharge characteristics called S-characteristics. In particular, the S-characteristics are conspicuous in the pump turbine suitable for the high pumping head.
The discharge characteristics of the pump turbine are generally expressed by a group of characteristics curves representing the relationship between a unit speed (N1=N/√{square root over (H)}) and a unit discharge (Q1=Q/√{square root over (H)})) by using a wicket gate opening as a parameter. A turbine operating region includes a part (a first part) at which the value Q1 is decreased as the value N1 is increased, and another part (a second part) at which the value Q1 is decreased as the value N1 is decreased. Furthermore, the first part is divided into a portion (a moderately variable portion at the first part) at which the value Q1 is relatively moderately decreased as the value N1 is increased, and another portion (a sharply variable portion at the first part) at which the value Q1 is relatively sharply decreased as the value N1 is increased. The sharply variable portion at the first part and the second part constitute an S-characteristics section.
In a generating mode of the pump turbine, a normal operation is performed at the moderately variable portion at the first part. However, since a wicket gate opening is about a no-load opening on starting, which is smaller than that in a normal operation with a load, and the value N1 exhibiting the S-characteristics also becomes small, an operating point not only relatively approaches the S-characteristics section but also intrudes into the sharply variable portion at the first part of the S-characteristics section in some cases. In particular, it becomes the most severe when synchronizing and paralleling of a pumping-generating plant with the power line is required at a lowest head at which the unit speed N1 becomes highest. Needless to say, since the speed is low at the beginning of the starting of operation, the unit speed N1 also is low, and therefore, is considerably apart from the S-characteristics section, thereby achieving stable acceleration. As the speed approaches a rated speed, the speed is started to be influenced by the S-characteristics. When the unit speed N1 falls within the first part of the S-characteristics section, a gradient ∂Q1/∂N1 becomes abruptly steep, and thus, discharge fluctuations with respect to speed fluctuations become great, that is, a water hammer becomes great, thereby deteriorating the stability of the speed government by a governor. Moreover, since the turbine discharge is increased from zero up to a no-load discharge on starting, the situation becomes severer with the influence of the resultant transient water hammer. As a consequence, the speed fluctuations cannot be suppressed at all around the lowest head, and therefore, the plant cannot be synchronized with or put on a power line in some cases. The situation in which the plant cannot be synchronized with or put on the power line signifies that the plant cannot be operated at that head, thus leading to an enormous loss. In the case of the pump turbine, as the head becomes greater, the runner must be formed into a flatter shape and in a larger diameter in order to exhibit desired pumping characteristics. Therefore, the unit speed N1 exhibiting the S-characteristics tends to be lower at the same wicket gate opening, thereby making the above-described problem more serious.